Heart rate monitor

Description: Light filtering through the fingertip is detected by the cadmium sulfide photoresistor CD1, which forms the feedback network for the transducer amplifier section ICA. This produces a weak signal that is further amplified by ICB. The amplified signal is then compared against a user-adjusted threshold, where comparator ICD triggers gating on the piezoelectric buzzer PZ1. On each falling edge of the comparator's output signal, one-shot multivibrator ICD produces a 2 µs pulse, which is inverted by Q1 and averaged by the RC network consisting of M1, C6, and C7. The trimpot R20 in Q1's collector circuit sets the scale factor for M1, where full scale is 150 beats per minute.

The circuit operates by detecting light changes caused by blood flow through a fingertip, utilizing a cadmium sulfide photoresistor (CD1) as the primary sensing element. This photoresistor is sensitive to variations in light intensity, which correlates with the volume of blood in the fingertip. The light signal is converted into an electrical signal through the transducer amplifier section (ICA), where it is initially weak and requires further amplification by the second amplifier stage (ICB).

The amplified output from ICB is then fed into a comparator (ICD), which compares the signal against a user-defined threshold. This threshold can be adjusted to suit different sensitivity requirements, allowing for customization based on the user's needs. When the signal exceeds this threshold, the comparator output changes state, triggering a piezoelectric buzzer (PZ1) to produce an audible alert.

The one-shot multivibrator (ICD) is configured to generate a precise 2 µs pulse each time the comparator output falls, ensuring that the signal is processed in a timely manner. This pulse is inverted by transistor Q1, which acts as a switch to control the subsequent averaging process. The RC network formed by components M1, C6, and C7 smooths the output signal, providing a stable representation of the detected pulse rate.

Furthermore, the trimpot (R20) connected to the collector of Q1 plays a crucial role in calibrating the system. It allows the user to set the scale factor for M1, enabling the system to accurately reflect heart rates up to a maximum of 150 beats per minute. This feature enhances the circuit's versatility, making it suitable for various applications, including health monitoring and fitness tracking. The overall design ensures reliable operation while maintaining user-friendliness through adjustable parameters.Light filtering through the finger tip is detected by the cadmium sulphide photoresistor CD1 which forms the feedback network for transducer amplifier section ICA producing a weak signal which is further amplified by ICB. This signal is now compared against a user adjusted threshold, comparator ICD triggers gating on the piezoelectric buzzer PZ1.

On each falligg edge of the comparator's output signal one-shot multivibrator ICD produces a 2 ^s pulse which is inverted by Ql and averaged by the RC network consisting of Ml, C6 and C7. The 10 trimpot R20 in Ql's collector circuit sets the scale factor for Mi where full scale is 150 beats per minute.